This application relates to technology for non-volatile data storage having reversible resistivity-switching behavior. In particular, this application relates to content addressable memory cells and memory arrays that include ReRAM devices and vertical transistors.
A variety of materials show reversible resistivity-switching behavior, and may be suitable for use in resistance-switching memory devices, sometimes referred to as “ReRAM” devices. Transition metal oxides exhibit reversible resistivity-switching behavior, and have been proposed for use in ReRAM devices. Upon application of sufficient voltage, current, or other stimulus, the reversible resistivity-switching material switches to a stable low-resistance state. This resistivity-switching is reversible, such that subsequent application of an appropriate voltage, current, or other stimulus can return the reversible resistivity-switching material to a stable high-resistance state. This conversion can be repeated many times. The low resistance state is sometimes referred to as an “ON” state, and the high resistance state is sometimes referred to as an “OFF” state. For some switching materials, the initial state is low-resistance rather than high-resistance.
ReRAM devices may be used in nonvolatile memory arrays. One type of memory array is referred to as a cross-point array, which is a matrix of memory elements typically arranged along x-axes (e.g., word lines) and along y-axes (e.g., bit lines). A digital value may be stored as a memory resistance (HIGH or LOW). The memory state of a memory cell can be read by supplying appropriate voltages to the bit line and word line connected to the selected memory element. The resistance or memory state can be read as an output voltage or current of the bit line connected to the selected memory cell. One resistance state may correspond to a data “0,” for example, whereas the other resistance state corresponds to a data “1.” Some switching materials may have more than two stable resistance states, and thus may be used to represent more than one bit of data.
ReRAM devices also may be used in content addressable memory (CAM). CAM cells typically include two complimentary memory elements with built-in XOR circuitry for fast matching of the stored data to a reference. CAM cells typically are made using CMOS SRAM cells or the like. Such existing CAM cells may require up to ten transistors, and thus can be quite large. CAM cell architectures with two ReRAM devices and seven transistors implemented in a CMOS process have been proposed.